Final answer:
A milestone is a significant event in project management that indicates a major achievement within a project and helps measure progress.
Step-by-step explanation:
The correct answer to the question is a) Milestone. A milestone in project management represents a significant event that marks a major achievement within a project or that is imposed upon the project. It is a key point in the project's timeline that marks the completion of a major task or the achievement of an important goal. Milestones help track progress and ensure that the project stays on schedule. It is often used to measure progress toward the overall goal. Milestones are not only measurable, indicating how far along a project is, but they are also motivational, providing the project team with visible targets to reach and celebrating along the way. This can be especially important in maintaining team momentum and morale.